A sonata is a piece of classical music in three or four separate parts, called movements, written either for a piano alone or for one other instrument with a piano — a violin sonata, a cello sonata, a flute sonata. The movements differ in speed and mood: often fast, then slow, then fast again, so the whole thing feels like a journey rather than one single song. Beethoven's 'Moonlight Sonata' is the one almost everyone has heard. A word of warning about a related term: sonata form is not the same thing. Sonata form is a plan for building ONE movement — you present two ideas, argue with them in the middle, then bring them back — and it is used inside symphonies and string quartets too, not only inside sonatas. Musicians keep these apart carefully. Historically the word simply meant 'a played piece', as opposed to a cantata, 'a sung piece'; the strict three-or-four-movement shape came later, in the 1700s.